selftests/bpf: assert BPF kfunc default trusted pointer semantics

selftests/bpf: assert BPF kfunc default trusted pointer semantics

The BPF verifier was recently updated to treat pointers to struct types
returned from BPF kfuncs as implicitly trusted by default. Add a new
test case to exercise this new implicit trust semantic.

The KF_ACQUIRE flag was dropped from the bpf_get_root_mem_cgroup()
kfunc because it returns a global pointer to root_mem_cgroup without
performing any explicit reference counting. This makes it an ideal
candidate to verify the new implicit trusted pointer semantics.

Signed-off-by: Matt Bobrowski <mattbobrowski@google.com>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20260113083949.2502978-3-mattbobrowski@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>